Motion Tracking not working for Eyeball
The Follow Track constraint includes a 'Constraint to F-Curve' button - click that and Blender will convert your track into a series of keyframes. You can then use the Graph Editor to edit the resultant F-Curves for the object.

To edit a curve, right-click on one of its points and press L to select 'Linked' (ie, select the whole curve). Then use S to scale it (you can type a number to scale by a fixed amount if desired - eg, S10Enter) to scale up by a factor of 10. Repeat for each of the X, Y, Z channels.

Once you've scaled up your F-Curve you'll find that the motion of the empty used for the track is similarly affected, resulting in more motion for the eye.